Bump gunicorn from 19.4.5 to 19.5.0

Bumps [gunicorn](https://github.com/benoitc/gunicorn) from 19.4.5 to 19.5.0.
- [Release notes](https://github.com/benoitc/gunicorn/releases)
- [Commits](https://github.com/benoitc/gunicorn/compare/19.4.5...19.5.0)

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -1,5 +1,5 @@
 django>=1.11,<1.12
 django-debug-toolbar==1.8
-gunicorn==19.4.5
+gunicorn==19.5.0
 psycopg2==2.8.5
 whitenoise==3.3.1